Vaso-occlusive devices are surgical implants that are placed within an opening in the vasculature which is to be occluded, such as, for example, within an aneurismal cavity to form an embolus by blocking the flow of blood. Vaso-occlusive devices are typically delivered and placed at a selected site in the vasculature using a catheter in a minimally invasive procedure. In order to create an embolus, numerous coils are implanted in the site, e.g., an aneurysm, until an adequate density has been achieved.
Vaso-occlusive coils are usually constructed of a wire made of a metal or metal alloy wound into a helix. Such vaso-occlusive coils are typically manufactured to assume a certain shape upon discharge of the device from a distal end of a catheter into a treatment site. The shape of these coil is defined by the shape of the coil in a “free energy state,” that is a state where there are no outside forces acting on the coil. A variety of such vaso-occlusive coils are known. For instance, U.S. Pat. No. 4,994,069, issued to Ritchart et al., describes a vaso-occlusive coil that assumes a linear helical configuration when stretched and a folded, convoluted configuration when released from the catheter. The stretched condition is used in placing the coil at the desired site via passage through the catheter. The coil assumes a relaxed configuration—which is better suited to occlude the vessel—once the device is released from the catheter. Ritchart et al., describes a variety of secondary shapes including “flower” shapes and double vortices. Unlike vaso-occlusive coils used prior to that time, Ritchart et al. discloses using a coil that is relatively soft and is delivered to the site using a pusher within a catheter lumen. Upon discharge from the delivery catheter, the coil may undertake a number of random or pre-determined configurations useful to fill the site.
Known vaso-occlusive coils may be used for filling relatively small vessel sites, e.g., 0.5-6.0 mm in diameter. The coils themselves are described as being between 0.254 and 0.762 mm in diameter. The length of the wire making up the vaso-occlusive coil is typically 15 to 40 times the diameter of the vessel to be occluded. The wire used to make up the coils may be, for instance, 0.051 to 0.152 mm in diameter. Tungsten, platinum, and gold threads or wires are typically preferred. Such coils may be easily imaged radiographically, readily located at a well defined vessel site, and retrieved, if necessary.
In order for vaso-occlusive coils to be most effective, it is desirable for the coils to fill a peripheral shell of the aneurysm. Ideally, the coils fill the void in a complex, but semi-uniform manner. However, because numerous coils are implanted, it is desirable that the coils do not become overly intertwined or otherwise prevent additional coils from being inserted.